1. Field of the Invention
The present invention relates to a wire-cut electric discharge machine which has a wire electrode cutting function, an automatic wire connecting function, and a disconnection restoring function.
2. Description of the Related Art
A wire electrode cutting operation which is performed in automatic wire electrode connection in wire discharge machining is performed to reproduce an end part of a wire electrode which has a smooth wire electrode surface which is necessary for the connection of the wire electrode during an operation of machining a workpiece. The wire electrode having the smooth wire electrode surface is obtained by cutting and removing a disconnection part of the wire electrode, a bent part of the wire electrode, or a scratched part of the surface of the wire electrode.
To perform the wire electrode cutting operation by annealing is a known technique as disclosed in Japanese Patent Application Laid-Open No. 6-304819, for example. In this technique, after a wire electrode to be annealed is clamped by a pair of energizing electrodes (a chuck part electrode and a detecting part electrode), the wire electrode is energized. Then, cutting tension is generated on the wire electrode, which is softened by heat generated by electric resistance, in a rewinding direction by a break roller and at the same time, tensile cutting is performed while maintaining the heat-generation part on a given position close to a nozzle outlet by cooling air or cooling water inside of an upper pipe. Thus, the cutting operation is completed.
Here, problems of the related art technique in which a cutting operation of a wire electrode is performed by annealing are described with reference to FIGS. 9 to 14.
FIG. 9 illustrates a previous stage of a start of cutting of a wire electrode. As the previous stage of a start of cutting of a wire electrode 11, air blowing is started so as to clean the wire electrode 11 and the inside of an upper pipe 21, as shown in FIG. 9. At this time, a chuck part electrode 20a on an energizing side and a chuck part electrode 20b on a clamping side are in such state that they do not clamp the wire electrode 11. Further, a pair of detecting electrodes 22 constituting a detecting part is in such state that the pair does not clamp the wire electrode 11, as well.
FIG. 10 illustrates an energization start for annealing. When the energization of the wire electrode 11 is started, the wire electrode 11 is clamped by the chuck part electrodes 20a and 20b and is clamped by the detecting electrodes 22 and 22 as illustrated in FIG. 10. Here, the chuck part electrodes 20a and 20b clamp the wire electrode 11 with a relative movable force with respect to the wire electrode 11 in a state that imparting of anneal torque by a break roller 18 is started and the wire electrode 11 is pulled. Then, current for annealing is started to be fed to the chuck part electrodes 20a and 20b and the detecting electrodes 22 and 22 via the wire electrode 11. This current feeding start is called “annealing energization start” hereinafter.
The wire electrode 11 generates heat and is softened by energization. Simultaneously with the annealing energization start, tensile force is imparted to the wire electrode 11 in a rewinding direction by rotary torque of the break roller 18. The tensile force imparted simultaneously with the annealing energization start is called “annealing torque” hereinafter. Here, during the energization for annealing, air supply for cleaning the inside of the upper pipe 21 is stopped and the wire electrode 11 is cooled down by cutting air so as to control a cutting position and straightness of the wire electrode 11. The flowing amount per unit time of the cutting air for cooling down to straighten the wire electrode 11 is smaller than that of the cleaning air.
FIG. 11 illustrates a process from annealing start to cutting. In order to increase tensile force which is imparted to the wire electrode 11, the annealing torque is switched to the cutting torque, as shown in FIG. 11. The cutting torque is imparted by controlling the rotary torque of the break roller 18 so that the wire electrode 11 is pulled in the rewinding direction, as is the case with the annealing torque. The cutting torque is larger than the annealing torque. Further, the cleaning air for cleaning the inside of the upper pipe 21 and the wire electrode 11 is switched to the cutting air. The wire electrode 11 which is annealed and softened is further stretched by the cutting torque and at this time, the wire electrode 11 rubs against the chuck part electrode 20a on the energizing side and the chuck part electrode 20b on the clamping side. Thus, the wire electrode 11 rubs against the chuck part electrodes 20a and 20b. Accordingly, wire metal powder derived from the wire electrode 11 is attached to surfaces of the chuck part electrodes 20a and 20b. 
Then, the wire electrode 11 is wound up by the cutting torque and is cut at a part on the electrodes of the detecting part. Almost all part of the wire electrode 11 between the chuck part electrodes 20a and 20b and the electrodes of the detecting part 22 penetrates through the inside of the upper pipe 21 and is cooled down by the cutting air which flows inside the upper pipe 21. Therefore, the wire electrode 11 is not cut inside the upper pipe 21 and is cut at a part on the electrodes of the detecting part 22.
FIG. 12 illustrates a process from a cutting end to a connection start of the wire electrode 11. When the cutting of the wire electrode 11 is ended, wire feeding of the wire electrode 11 is started in a winding direction 15. At this time, the wire electrode 11 rubs against the chuck part electrodes 20a and 20b, so that wire metal powder is attached to the surfaces of the chuck part electrodes 20a and 20b. 
In the above-described technique, when the wire electrode 11 which has been annealed is pulled in a rewinding direction 14 by the annealing torque of the break roller 18, the wire electrode 11 generating heat rubs against the surfaces of the chuck part electrodes 20a and 20b which clamp the wire electrode 11 for energization and thereby, the wire metal powder is attached to the surfaces of the chuck part electrodes 20a and 20b. Therefore, if the cutting operation of the wire electrode 11 is repeated a plurality of times (approximately 200 to 300 times), wire metal powder is piled up on the surfaces of the chuck part electrodes 20a and 20b and thereby, the surfaces become rough. If the cutting operation of the wire electrode 11 is performed by the chuck part electrodes 20a and 20b in such state, the wire electrode 11 does not tightly contact with the surfaces of the chuck part electrodes 20a and 20b when current is fed for cutting the wire electrode 11, that is, the contact state between the wire electrode 11 and the surfaces of the chuck part electrodes 20a and 20b is unstable, as shown in FIG. 13. Consequently, chattering occurs and minute discharge frequently happens.
A large number of discharge traces are formed on the surfaces of the chuck part electrodes 20a and 20b due to the minute discharge. As a result, deep scratches of the discharge traces which have been formed on the surfaces of the chuck part electrodes 20a and 20b are transferred to the surface of the wire electrode 11 which has been annealed. The wire electrode 11 to which deep scratches of discharge traces have been transferred causes such problem that scratched part of the wire electrode 11 is frequently clogged at the wire guide parts of an upper die guide 24 and a lower die guide 25 in a wire connecting operation as shown in FIG. 14, substantially decreasing a connection success rate. Especially, when the wire electrode 11 is a soft wire, this problem is conspicuous. Further, though certain amount of wire metal powder is inevitably attached on the surfaces of the chuck part electrodes 20 in a wire cutting device of an annealing method, the device becomes unusable after about 200 to 300 times of cutting in a case where discharge traces have been formed due to minute discharge. Thus, device life is shortened.